How Can I calculate bank of resistor to eletric motor whit winding rotor???

[Cbeltranochoa]

I don´t know, How Can I Calculate bank Of resistor to electric motor whit winding rotor

Can you help me????


Thanks.

 

[ScottyUK]

Do you mean that you have a wound rotor motor, i.e. a motor with sliprings, and you want to calculate the size of the external resistor for the motor? Do you have any more information about the motor, or a photograph of the nameplate?

 

[crshears]

If a wound rotor motor is in fact what the OP is inquiring about, the use of starting resistors for these is so yesterday...

Eugene C. Lister's "Electric Circuits and Machines," Sixth Edition, McGraw-Hill, C. 1984, pgs. 398-399, gives a general description of how industrial electronics can be harnessed to provide smooth wound-rotor motor starting while incorporating highly efficient slip power recovery and regenerative speed control.

In other words, it's been more than 25 years since starting resistors became obsolete; it's time to move on.

 

[dArsonval]

Acknowledging Wound Rotor Motors are "prior Art" from another time, I'm curious as well.
How does... one calculate the resistance required?

There is... indeed... a trend to retire slip-ring motor applications... and yet... they are still out "here".

Is there a "boiler plate" answer to this post?

Always enjoying this forum,

John

 

[ScottyUK]

Resistor systems will work long after the electronics has either failed or been made obsolete. In severe environments the simple, rugged, reliable, field-maintainable solution may be worth the efficiency loss.

However! We are derailing the OP's post and he's clearly struggling with the English language so let's not make things more difficult for him than they already are.

 

[LionelHutz]

The easy way is to call a resistor manufacturing company and give them the rotor rated voltage and current and number of starting steps desired and they will provide a quote.

The hard way is to use these 2 facts and and calculate them yourself. The rotor voltage is inversely proportional to speed or proportional to slip. The rotor current is proportional to torque output. Pick the number of steps and then calculate the resistor needed to bring the motor to the maximum speed for each step.

The EASA Technical Manual has a section on calculating the rotor resistors to start a motor. The basics are to take the secondary phase voltage and divide by phase current to calculate the total rotor resistance. Don't use the phase-to-phase voltage that is typically given on the nameplate - divide that by sqrt(3) first. Use the following table to calculate the starting resistance steps as percentages of this total resistance.
2 steps - 70 & 30
3 steps - 55 & 30 & 15
4 steps - 40 & 30 & 20 & 10
5 steps - 34 & 26 & 19 & 13 & 8

 

[Cbeltranochoa]

these are the details of my electric Motors:

Electric Motor with winding rotor
3 Phases
KW: 15 Kw
stator Voltage : 440 Volts
Stator Amperes: 33 Amperes

Frecuency: 60 Hz

855 RPM

Voltaje Rotor : 187 Volts
Amperes Rotor: 55 Amperes

 

[LionelHutz]

Sorry, we're not doing it for you. 187V is the line to line rotor voltage and 55A is the current. Use what I posted and do the math.

 

[ScottyUK]

Is that because you, uh, can't...?

Don't worry, neither can I - not without digging in to some very old texts to help. They're at work and I'm on holiday until next year.

 

[LionelHutz]

We don't even know if he wants speed control or just to start the motor to full speed so how can we calculate anything with any certainty?

I posed how to calculate the resistors for motor starting. It's rather simple to do. I'm not providing resistor numbers to the original poster unless he understands what they are for. At that point, he can calculate them himself.

 

[ScottyUK]

Agreed.

 

[7anoter4]

I think the total rotor resistance and the reactance has to be taken into consideration.
If rpm[mec]=855 wsync=900 then sn=0.05
Since , Pgap=3*Irotor^2*Rrotor/s then Rrotor=Pgap*sn/3/Irotor^2
Pgap=Pmec/(1-sn)=15789 w
Irotor=55 A will get Rrotor=0.08699 ohm.
But Irotor=Erot*sn/sqrt(Rrotor^2+Xrotor^2*sn^2) then Xrotor=sqrt(Vrot^2*sn^2/3/Irot^2-Rrot^2)/sn=0.9089 ohm
This wound rotor resistance and reactance usually does not present big difference from start up to rated rpm. Then for each step the rotor current will be:
Irotor=s*Vrot/sqrt(3)/sqrt((Rrot+Rext)^2+Xrot^2*s^2)
Pgap=3*Irot^2*(Rrot+Rext)/s
Let's take Rext total=1.7 ohm and let's take 5 steps as follows:
rotor current will be:
Irotor=s*Vrot/sqrt(3)/sqrt((Rrot+Rext)^2+Xrot^2*s^2)
Pgap=3*Irot^2*(Rrot+Rext)/s
Let's take Rext total=1.7 ohm and let's take 5 steps as follows:

 

[7anoter4]

I’m very sorry! Something is wrong in my calculations! DON’T TAKE IT AS SHOWN IN THE ATTACHEMENT. I have to check my program.

 

[Cbeltranochoa]

Dear

7anoter4.

Ok thanks, the most important is your intention

Thanks


Best Regards

 

[PersonalProfile]

I suspect you are familiar with the "equivalent" circuit of a motor.
Personally, I have achieved what I needed to achieve (not necessarily what you need to, though it sounds somewhat similar) through creating an equivalent circuit of the motor and adjusting the "R" of "R2/s" as required / observing the impact on the key parameters.

Regards,
Lyle